In the production of aluminum alloy products, aluminum is melted, alloyed and cast into ingots which are then subjected to a variety of working operations such as rolling, extruding, milling, etc. Processes used for casting aluminum alloy ingots include direct chill continuous casting and electromagnetic casting.
In direct chill casting, molten aluminum is poured into a shallow water-cooled mold of a desired cross-sectional shape to form an ingot. When the metal begins to freeze in the mold, a false bottom in the mold is lowered at a controlled rate and water is sprayed on the surface of the freshly solidified metal as it comes out of the mold.
In electromagnetic casting, molten aluminum is fed to a mold and confined by electromagnetic forces therein. The molten aluminum is solidified and withdrawn from the mold to form a cast ingot.
A particular concern involved in the process of continuous casting aluminum alloys into ingots is avoiding the formation of metal oxides and preventing slag, dross or other impurities from entering the ingot mold to adversely effect the surface quality of the ingot being cast. Surface imperfections in the ingot sidewalls result in higher operating costs since the cast ingots require surface treatment such as scalping or the like to remove surface imperfections before further working is done.
The prior art has proposed various solutions to improve the cast surface quality of aluminum alloys. U.S. Pat. No. 3,779,389 discloses a bag-shaped filter which is designed to remove solid contaminants from aluminum during casting. The bag-shaped filter is arranged in the ingot head to filter solid particulate material from the molten metal.
U.S. Pat. No. 3,926,690 discloses an aluminum-iron-silicon alloy having strontium and/or calcium as an alloying component thereof to reduce pickup during the extrusion of a cast ingot.
U.S. Pat. No. 4,523,627 discloses an electromagnetic casting method characterized by the use of a top feeder head in combination with an electromagnetic field. Controlling the position of the height of the top feeder head and the casting cooling system permits casting of aluminum and alloys thereof in the form of billets or plates which have surfaces that do not require any scalping treatment.
However, conventional continuous casting of aluminum alloys into ingot form does not consistently provide an acceptable surface quality on the cast ingots such that the cast ingots can be directly worked without the need for scalping or other surface treatment. In particular, the production of can body stock alloys, such as AA3104, and can end stock alloys, such as AA5182, require a high quality ingot surface before further working is performed. Aluminum alloys of the AA5000 series, such as AA5182, are particularly difficult to electromagnetically cast without producing significant surface imperfections in the ingot surface requiring scalping treatment. It is believed that the presence of magnesium oxide in the vicinity of the ingot surface during casting is a primary component that contributes to poor surface quality in the as-cast product.
In view of the deficiencies in the prior art discussed above, a need has developed to provide an improved electromagnetic casting process for aluminum and aluminum alloys which produces an ingot product having a surface quality sufficient to permit further working of the cast ingot without the need for a surface treatment such as scalping or the like.
In response to this need, the present invention provides an improved electromagnetic casting process which produces a cast aluminum or aluminum alloy ingot having an ingot surface quality which is generally free of surface imperfections. The cast ingot can be directly worked, rolled or reduced in size without the need for a surface conditioning or scalping operation.